1. Field of the Invention
The present invention relates to an improved drill bit and to a method for making the same and more particularly to such a drill bit of the solid body type having cutting elements embedded therein.
2. Setting of the Invention
In the drilling of oil and gas wells, rotary motion and downward force are applied to a drill bit to create a wellbore. The bit may be mounted on the lower end of a string of drill pipe which is rotated at the surface of the well or it may be mounted on a downhole hydraulic motor which in turn is suspended from a drill pipe string. In both types of drilling, the drill bit usually includes openings in the lower end thereof to enable circulation of drilling fluid down the pipe string, through the bottom of the bit, and upwardly into the annulus between the outer surface of the pipe string and the wellbore. The drill bit is rotated while the fluid circulation flushes cuttings from the bottom of the wellbore and cools the bit.
One type of drill bit which is used as described above includes a solid body having cutting elements embedded therein. Often the cutting elements on such drill bits are diamonds embedded in a lower curved surface on the bit. As the bit rotates, the diamonds cut the formation thereby increasing the depth of the bore. Friction of the diamonds sliding against the formation heats the diamonds. Diamond wear increases exponentially as the temperature of the diamond increases. It is important that drilling fluid flow around each diamond during drilling to minimize its temperature and thus its wear. Cuttings which adhere to the diamonds impede heat transfer so it is important to flush cuttings out of the bottom of the bore into the annulus.
In a conventional diamond drill bit, drilling fluid exits the center of the bit at the lower end thereof and flows toward the annulus between the drill string and the wellbore in a narrow gap between the lower end of the bit and the bottom of the bore. In such bits, the great majority of the hydraulic energy of the fluid is expended in the gap toward the center of the bit and very little of the energy is expended on the radially outer portions of the bit. The hydraulic energy or horsepower can be expressed as a product of the flow rate of the drilling fluid and the pressure drop. It can be theoretically proven that the cooling of each diamond is directly related to the hydraulic horsepower expended around the diamond. Thus, in a conventional drill bit, the diamonds toward the center of the drill bit are more than adequately cooled while the diamonds on the radially outer portions of the bit are inadequately cooled and thus subject to rapid wear. Since little of the hydraulic energy is expended on the outer portion of the bit, cuttings are more likely to adhere to the diamonds on the outer bit portions.
Conventional solid body drill bits suffer from other disadvantages. When such conventional bits are mounted on a downhole hydraulic motor, fluid is circulated down the drill string and through the motor thereby rotating the drill bit. Fluid continues through the bit out the bottom thereof and into the annulus. As the fluid flows from the drill string, through the bit and into the wellbore beneath the bit, an upward force is generated by the pressure differential between the inside and the outside of the bit. This force is directly related to the pressure drop beneath the bit and tends to reduce the weight applied to the bit as it increases. This force is commonly referred to as the "pump-off effect."
In copending applications filed by the inventors of the instant invention, description is made of the manner in which the pump-off effect is used to regulate the penetration rate of the drill bit. Such regulation is achieved by venting a selected amount of drilling fluid into the annulus rather than circulating it through the bit. It has been discovered that the greatest control of the pump-off force is achieved with bits that have the most pump-off force per unit of pressure drop across the bit. A bit which has a uniform distribution of hydraulic energy across its face will also have a high pump-off force per unit of pressure drop.
There exists a need for a solid body drill bit which has a uniform distribution of hydraulic energy across its face during drilling in order to uniformly cool the cutting elements on the face of the bit and to flush cuttings from beneath the bit.
There also exists a need for a drill bit having a high pump-off force per unit of pressure drop across the bit for use in drilling methods and apparatus in which the pump-off effect is used to control the penetration rate of the drill bit.